Former party chairman, Nebraska senator among those who say they won’t vote for him if he is nominee

A divisive battle is brewing in the Republican Party over the potential nomination of Donald Trump, as some party leaders warn they won’t back him and could support third-party or write-in candidates.

Most party leaders still say they will back the party’s nominee, and until recent days even Mr. Trump’s loudest critics maintained they would back him in the general election if he wins the nomination.

That...

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